<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin:0;
            padding:0;
        }
        
        html {
            font-size:5px;
           
        }

        body {
            height:300px;
        }
        .box {
            width:1rem;
            height:1rem;
            background:pink;
        }
        
        .embox {
            font-size:10px;
            width:300px;
            height:100%;
            background:yellow;
            /* height:300px; */
        }
        .embox-box {
            /* font-size:100px;
            width:100%;
            height:100%;
            background:red; */
        }
        

        .mask {
            position: fixed;
            top:0;
            left:0;
            width:100vw;
            height:100vh;
            background:rgba(0, 0, 0, 0.27);
        }
    </style>
</head>
<body>
    <div class="mask">

    </div>
    <div class="box">

    </div>

    <div class="embox">
        <div class="embox-box"></div>
    </div>
    

</body>
</html>